1. Field of the Invention
The present invention relates to a dry etching method, and more particularly to a dry etching method for simultaneously etching a plurality of silicon regions different in impurity concentration from each other.
2. Description of the Prior Art
When a monocrystalline or polycrystalline silicon wafer is etched by a conventional dry etching method such as plasma etching or reactive ion etching, there arises a problem that the etching rate and the shape or dimensions of a pattern formed by the etching are different in regions of the silicon wafer having different characteristics. That is, the etching rate for the silicon wafer varies with impurity concentration thereof. In the case where, after an impurity such as phosphorous, boron, or arsenic has been diffused in or implanted through the ion implantation technique in a desired portion of a monocrystalline silicon substrate or a polycrystalline silicon film, the silicon substrate or silicon film is subjected to dry etching to fabricate a semiconductor device, the etching rate is different in a plurality of regions of the silicon substrate (or silicon film) which have different impurity concentrations.
Accordingly, in the case where a plurality of regions in a silicon wafer which are different in impurity concentration from each other, must be etched simultaneously, it has been difficult to etch these regions to the same depth at the same period of time, and the etching depth (namely, the degree of etching) differs in regions.
Further, according to the usual plasma etching method, after the etching in the direction of depth has been completed, side etching proceeds, and thus the width of a pattern formed by etching becomes small. Accordingly, in the case where impurity concentration is different in regions of a silicon wafer, patterns formed in these regions are different from each other in dimensional accuracy.